Radar level gauge systems for measuring the level of a liquid or other filling materials in a tank are well-known, and such a system generally comprises a transmitter for transmitting a microwave signal towards the surface of the liquid, a receiver for receiving the microwave signal reflected against the surface of the liquid, and a signal processing device for calculating the level of the liquid in the tank from the propagation time of the transmitted and reflected microwave signal. Such device has become more and more important, particularly for petroleum products such as crude oil and products manufactured from it. The tank may be a large container constituting parts of the total loading volume of a tank ship, or even larger usually circular-cylindrical land-based tanks with volumes of tens or thousands of cubic meters.
In some applications the tank may contain interior structures causing disturbing reflections. In other applications, the tank may have a moving roof, adapted to be located close to the surface. In both these situations, it may be advantageous to provide the gauge with a wave guiding structure, typically a so called “still pipe” with a circular cross-section, extending from the antenna into the contents of the tank.
The most suitable propagation mode in such a wave guiding structure is the H01 mode. In order to generate electromagnetic radiation in this mode, a circuit board with a patch array feeder may be used. The radiator elements (patches or slots) are fed and shaped to create a circumferential electrical field typical for the H01 propagation mode. However, for high frequencies, such a feeder becomes difficult to manufacture due to the extreme tolerances. At the same time, a high frequency band, such as the 26 GHz band, is very suitable for radar level gauging purposes, due to the high accuracy it offers. Also for practical reasons, it is desirable to use the same type of electronics units for applications with still pipes as well as other applications. Consequently, there is s need for an H01 mode generator which is easy to adapt to higher frequencies.
It is well known to use optical elements in the design of antennas, and examples include a horn antenna with a lens over its opening and various reflector antennas. Especially pencil-beam antennas for higher frequencies are easier to manufacture using a basically optical design.
One example of a mode generator in a still pipe is shown in U.S. Pat. No. 4,641,139. According to this design, a curved reflecting surface is provided in the cross section of the still pipe, and another curved surface is provided a bit further down. The lower surface is structured with a conducting radial pattern, making it transmissive only to a circumferential mode. The upper surface is structured with a conducting spiral shaped “twist” pattern, where each lead forms a 45 degree angle with a radius. The spiral pattern is backed by a full metallic surface a quarter of a wavelength behind it and the combination will turn a radial field to a circumferential and vice versa. The combination of a twist-reflector and a sheet transparent for one reflection is known as “trans-twist” reflector and used in many reflector antennas for linear polarized waves.
Radial electromagnetic radiation (E01-mode) from an annular radiator will be reflected by the lower surface towards the upper surface. At reflection by the upper surface, the spiral pattern will turn the radial E01-mode to a H01 mode with a circumferential electrical field. The H01 beam will now be transmitted by the radial pattern in the lower surface, and be emitted into the tank. The curvature of the surfaces creates a focusing effect upon reflection, ensuring that parallel beams are emitted into the tank throughout the cross section of the pipe.
While providing several advantages, the solution according to U.S. Pat. No. 4,641,139 requires very small scale structuring of the curved surfaces, and especially the thin plastic sheets may be challenging to manufacture. Further, the structural strength of the lower surface may represent a problem, and any movement in this surface will influence the accuracy of the gauge.